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PLANT PHYSIOLOGY 



Experiments with transpirographs are usually successful only in 

 a closed room, such as a greenhouse. When they are conducted 

 out of doors, wind interferes with the accuracy of the records. 

 In order to compare the results obtained by different experi- 

 ments, it is necessary to reduce them to a common standard unit. 

 Most frequently, the total quantity of water lost by a plant 

 during a certain period is expressed per unit of its leaf surface. 



The magnitude thus obtained is called the 

 '' intensity of transpiration" and is usually 

 expressed in grams per hour per square meter 

 of leaf area. Sometimes, instead of the use of 

 the surface area, which in highly dissected 

 leaves of irregular shape is difficult to determine, 

 the green or even th^ dry weight of the leaves is 

 used in calculation. In comparing the amount 

 of water lost per ,unit time with the total 

 amount of water contained in the plant, the rate 

 of expenditure of the water reserve, expressed in 

 percentage, is obtained. In comparing the 

 amount of water lost by the plant during a 

 rather long period, for instance, several weeks 

 or even the entire growth period, with the 

 amount of dry matter accumulated in this time, 

 the transpiration efficiency, expressed in grams 

 per kilogram of water lost, is obtained. The 

 reverse value, the number of grams of water 



Fig. 94. — Method i • i j.' i r j u j. 



of determining the ^^^^ ^^ accumulatmg 1 g. of dry substance, is 

 water used by a termed the 'transpiration coefficient." Some 



authors, chiefly American, call this value the 

 *' water requirement" of the plant. 



An idea of the intensity of transpiration is obtained by com- 

 paring the rate of transpiration from a unit leaf surface with 

 the rate of evaporation from a unit area of a free water surface. 

 The magnitude obtained is called '' relative transpiration." It 

 shows to what extent the transpiration of a leaf is slower when 

 compared with ''free evaporation." 



All these magnitudes vary greatly in different plants and under 

 the influence of different environmental conditions. In most 

 plants, the intensity of transpiration ranges from 15 to 250 g. 

 per square meter per hour in daytime, and from 1 to 20 g. at 



